为什么动态链接器减去虚拟地址以找出加载的共享库可执行文件在内存中的位置？

Question

根据dl_main中的ld源代码here。如果没有 phdr 传递给 dl_main 的上下文，我对为什么 main_map 的加载地址是通过减去虚拟地址推导出来有点困惑。

我查过的代码：

1124 static void
1125 dl_main (const ElfW(Phdr) *phdr,
1126          ElfW(Word) phnum,
1127          ElfW(Addr) *user_entry,
1128          ElfW(auxv_t) *auxv)
1129 {
1130   const ElfW(Phdr) *ph;
1131   enum mode mode;
1132   struct link_map *main_map;
1133   size_t file_size;
1134   char *file;
1135   bool has_interp = false;
1136   unsigned int i;
1137   bool prelinked = false;
1138   bool rtld_is_main = false;
1139   void *tcbp = NULL;
...    // Before this else, it thinks you're calling `ld.so.<version>` directly. This is not usually the case.
1366   else
1367     {
1368       /* Create a link_map for the executable itself.
1369          This will be what dlopen on "" returns.  */
1370       main_map = _dl_new_object ((char *) "", "", lt_executable, NULL,
1371                                  __RTLD_OPENEXEC, LM_ID_BASE);
1372       assert (main_map != NULL);
1373       main_map->l_phdr = phdr;
1374       main_map->l_phnum = phnum;
1375       main_map->l_entry = *user_entry;
1376 
1377       /* Even though the link map is not yet fully initialized we can add
1378          it to the map list since there are no possible users running yet.  */
1379       _dl_add_to_namespace_list (main_map, LM_ID_BASE);
1380       assert (main_map == GL(dl_ns)[LM_ID_BASE]._ns_loaded);
1399     }
...    // Loops through program headers loaded in sequence from the ELF header.
1409   for (ph = phdr; ph < &phdr[phnum]; ++ph)
1410     switch (ph->p_type)
1411       {
1412       case PT_PHDR:
1413         /* Find out the load address.  */
1414         main_map->l_addr = (ElfW(Addr)) phdr - ph->p_vaddr;
1415         break;

那么这里为什么要减去加载地址呢？

Answer 1

特别是在第 1414 行，我们看到 main_map->l_addr = (ElfW(Addr)) phdr - ph->p_vaddr;。从为 main_map 定义类型 link_map 的文件 link.h 中，我看到 link_map 是描述已加载共享对象的 struct。 l_addr 字段用于描述当您运行 readelf:

❯ readelf -l main

Elf file type is EXEC (Executable file)
Entry point 0x401020
There are 11 program headers, starting at offset 64

Program Headers:
  Type           Offset             VirtAddr           PhysAddr
                 FileSiz            MemSiz              Flags  Align
  PHDR           0x0000000000000040 0x0000000000400040 0x0000000000400040
                 0x0000000000000268 0x0000000000000268  R      0x8
  INTERP         0x00000000000002a8 0x00000000004002a8 0x00000000004002a8
                 0x000000000000001c 0x000000000000001c  R      0x1
      [Requesting program interpreter: /lib64/ld-linux-x86-64.so.2]
  LOAD           0x0000000000000000 0x0000000000400000 0x0000000000400000
                 0x00000000000004c0 0x00000000000004c0  R      0x1000
...

这意味着 l_addr 不是 load 地址。它实际上是您需要从当前内存中添加以访问共享对象内容的 offset。

为什么动态链接器减去虚拟地址以找出加载的共享库可执行文件在内存中的位置？

Why does the dynamic linker subtract the virtual address to find out location of loaded shared library executable in memory?

c++

elf

ld

为什么动态链接器*减去*虚拟地址以找出加载的共享库可执行文件在内存中的位置？

Why does the dynamic linker *subtract* the virtual address to find out location of loaded shared library executable in memory?

c++

elf

ld

为什么动态链接器减去虚拟地址以找出加载的共享库可执行文件在内存中的位置？

Why does the dynamic linker subtract the virtual address to find out location of loaded shared library executable in memory?